Semiconductor wafers are commonly used to produce integrated circuits and other microelectronic devices. These wafers are typically disks made of fragile materials that are relatively thin as compared to their surface areas. For example, a 300 mm diameter silicon wafer can be approximately 0.775 mm thick. These wafers are often subjected to several processing steps during the manufacturing of microelectronic devices, during which it is important both to keep the wafer extremely clean and to protect the wafer surfaces from being scratched, broken, or otherwise damaged.
Many of the steps involved in semiconductor wafer processing include placing a wafer or group of wafers in a fluid processing tank where the wafers are exposed or subjected to various processing solutions to achieve particular results, such as chemical etching of wafer surfaces. Common processing fluids that are used include acidic solutions, basic solutions, and highly oxidative solutions. After each processing step, any contaminants and/or processing fluids remaining on the surface of the wafer are typically rinsed in a separate rinsing step to prepare the wafer for further processing or as a final step in the processing of the wafer. The processing and rinsing solutions may be sequentially applied in the same processing tank in a process where the wafers are placed in a tank to which fluids are added then drained for each sequential treating and rinsing operation. Alternatively, wafers may be transferred from one processing tank to another, where the different solutions can be sequentially applied to the wafers in the different processing tanks. An additional processing step that may be required between subsequent processing steps (such as after a rinsing process) is referred to as a drying step, in which any droplets or films that remain on a wafer surface are removed from that surface with equipment such as an isopropyl alcohol (IPA) vapor dryer. This additional drying step can be particularly beneficial to remove droplets containing contaminant particles before those particles are deposited on a wafer surface due to evaporation of that droplet.
When processing a group or set of wafers, it is important that each wafer be sufficiently spaced from adjacent wafers to allow an adequate flow of the various processing and rinsing solutions, thereby allowing for uniform surface treatments to reach all surfaces of each wafer. In addition, it is important that each wafer be held as close to its outer perimeter as possible throughout the various processing and rinsing steps in order to provide the processing and rinsing fluids with unobstructed access to the critical surfaces of each wafer. In fact, current wafer processing guidelines allow for handling only the outer 3 mm of each wafer, which is considered to be the unusable portion of the wafer and may also be referred to as the exclusion zone of the wafer. To provide additional usable area of each wafer, it may be desirable to handle each wafer even closer to its edges, such as the outer 1 mm of each wafer, for example.
The two basic techniques commonly used for holding wafers throughout multiple processing steps include systems with wafer supports or cassettes and systems that do not use cassettes. One example of a commonly used cassette-based system is illustrated schematically in FIG. 1, in which a group of semiconductor wafers is being cleaned using a fluid and megasonic energy. This cleaning system 10 includes a tank 12 containing a wafer or group of semiconductor wafers 14, where the group of wafers 14 is supported by wafer supports 16. In this example, the group of wafers 14 is supported by four wafer supports 16 that are either part of a cassette that moves along with its group of wafers 14 between multiple tanks for various processing operations or part of a wafer support system that remains in the tank when the wafers are withdrawn using a tank-to-tank robot or other wafer removal device.
In wafer processing systems that do not use cassettes, multiple wafers are typically moved and held as a group by more than one type of mechanism, where the mechanisms do not remain with a particular group of wafers for more than one step of the process. For example, a group of wafers can be transferred from one processing tank area to another processing tank area by a tank-to-tank robot, and separate elevator robots can then remove the wafers from the tank-to-tank robot and lower them into a processing tank. These elevator robots can support the wafers in the same general manner as that described above for supporting wafers during both the transportation of wafers and the processing of wafers within each tank. That is, the wafers in this type of configuration can rest on wafer supports that are arranged similarly to the wafer supports 16 of FIG. 1 and that are located along the bottom edge of the wafers. The wafer supports used in any of these arrangements may be provided with grooves or teeth between which the wafers can rest; however, the wafers are not actively held in place by any movable mechanisms.
In either of the system types described above, it is common to treat the wafers both with the addition of fluid to the tank and with the application of additional energy to the system to further assist in the removal of undesirable contaminants from the surfaces of the wafers. Referring again to FIG. 1, a megasonic transducer array 18 is used to introduce megasonic energy from the bottom of the tank 12. Because this energy comes from the bottom of the tank, the energy that comes from directly beneath the wafer supports 16 will be distorted as it moves upward and meets the wafer supports 16. Thus, the areas that are directly above the wafer supports 16 will be exposed to less of the megasonic energy than the remainder of the wafer, thereby lessening the effectiveness of the wafer cleaning in these areas. These areas may be referred to as “shadow” areas of the wafers, which can sometimes result in unusable areas of wafers or entire unusable wafers because they contain more than an allowable amount of contamination. Shadow areas can be reduced or eliminated with various techniques, such as, for example, applying a larger amount of megasonic energy to the wafers or applying megasonic energy from multiple sources. However, these more complicated and time-consuming techniques tend to reduce the efficiency and cost-effectiveness of the process and can result in damage to fragile wafer features or components due to overexposure to megasonic energy. Ideally, therefore, the entire wafer is exposed to a relatively equal amount of megasonic energy in only a sufficient amount to achieve the desired cleaning action.
One example of a cassette-based system that can be used to minimize the obstruction of megasonic energy that reaches the wafer is disclosed in U.S. Pat. No. 6,264,036 (Mimken et al.). The cassette of this patent is formed from two substantially parallel rods, each rod having notches spaced from one another for receiving objects to be carried. The cassette further includes a pair of supporting members extending between the rods, which are spaced apart so that substantially planar objects will fit between the rods and rest in their notches. Because the cassette is designed so that each side of a wafer positioned between the rods contacts each side of the cassette at one point that is approximately at the centerline of the wafer and at another point that is spaced slightly below the centerline, the wafer is basically primarily being held in place by gravity at the two points below the centerline on either side of the wafer. As described above, this cassette is designed to hold wafers as they travel throughout various processing steps, with the added advantage of limited obstruction of the megasonic energy that reaches the faces of the disk.
While the above described cassette can be advantageous to provide better fluid access to the disk surfaces, as the number of points that the wafer contacts a supporting surface decreases, the tendency of the wafers to tip toward and away from adjacent wafers increases. In particular, a cassette system with only two contact rods that are positioned relatively high on the wafer would typically be a less stable system than the system of FIG. 1 which includes four contact rods that are positioned much closer to the bottom of the wafer. In general, cassette-based systems can be disadvantageous because fluid from one tank can be carried on the cassette to another tank, which can thereby contaminate the fluid of the later tank. Thus, it is desirable to provide a system for holding wafers that minimizes obstruction of the wafer surfaces while keeping the disks relatively stable with respect to one another and the surrounding structure, and also minimizes fluid carry-over by moving the wafers without a cassette.